1. Field of the Invention
The present invention relates to a control method or system for controlling downshifting of an at least partially automated vehicular mechanical transmission. In particular, the present invention relates to a downshift control operating under logic rules which will sense a condition wherein the vehicle engine is unable to achieve an engine speed sufficient to allow synchronous engagement of the downshift target ratio and, in response thereto, will select the lowest speed ratio which can be synchronously engaged at the current vehicle conditions as a new, degraded mode target ratio.
2. Description of the Prior Art
Fully and partially automated vehicular mechanical transmission systems are known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,361,060; 4,595,986; 4,648,290; 4,722,248; 4,754,665; 5,406,861; 5,413,012; 5,441,463 and 5,441,464, the disclosures of which are incorporated herein by reference.
Such fully or partially automated vehicular mechanical transmission systems typically include a microprocessor-based controller for issuing command output signals to various controllers to implement selected transmission shifts, including manipulation of the engine fueling and/or engine or input shaft brakes to cause the transmission input shaft to rotate at a substantially synchronous speed for a given output shaft speed and target gear ratio. To accomplish a downshift in such a fully or partially automated mechanical transmission, the sequence of operations include automatic or manual selection of a downshift target ratio, disengagement of the currently engaged ratio, reengaging or retaining in the engaged condition the vehicle master clutch, causing the engine and, thus, the input shaft to rotate at a substantially synchronous speed for engaging the target gear ratio at current vehicle speed (usually as determined by output shaft speed) and, upon sensing attainment of synchronous speed, causing the jaw clutches associated with the target gear ratio to be engaged.
While the prior art fully or partially automated mechanical transmission systems are generally satisfactory, they are subject to improvement, as the systems tend to attempt to synchronize for engagement of a selected target gear ratio for an excessively long period of time under conditions wherein substantial synchronization could not be attained. Such conditions may include intentional or inadvertent application of full or partial engine braking, faulty engine fueling and/or control system communication and/or excessive vehicle speed and/or acceleration.